1. Field of the Invention
The present invention generally relates to image display devices and methods therefor, and, more particularly, the present invention relates to methods and devices for combining a reflection of a tomographic image with human vision during subcutaneous medical procedures.
2. Description of the Background
Because human vision depends at least partially on the detection of reflected visible light, humans cannot “see” into objects through which light does not pass. In other words, humans cannot see into the interior sections of a non-transparent, solid object. Quite often, and in many different technology areas, this sight limitation may impede or hinder the effective completion of a particular task. Various partial solutions to this problem have been utilized in the past (miniature cameras, x-ray methodologies, etc.). However, there is a continued need for improvement to the methods by which the interior of an object is displayed, especially using a real-time imaging modality.
Perhaps in no other field is this sight limitation more of a hindrance than in the medical field. Clinical medicine often calls for invasive procedures that commence at the patient's skin and proceed inward to significant depths within the body. For example, biopsy needles introduced through the abdominal wall to take samples of liver tissue for diagnosis of cancer must pass through many centimeters of intervening tissue. One potential problem with such procedures is the lack of real-time visual feedback in the vicinity of critical structures such as the hepatic arteries.
Standard imaging modalities such as Computerized Tomography (CT) and Magnetic Resonance Imaging (MRI) can provide data for stereotactic registration of biopsy needles within targets in the liver, lungs, or elsewhere, but these methods are typically characterized by the physical displacement of the patient between the time of image acquisition and the invasive procedure. Real-time imaging modalities offer more immediate feedback. Among such real-time modalities, ultrasound may be well-suited for guidance of needles because it preferably is relatively portable, is inexpensive, produces no ionizing radiation, and displays a tomographical slice, as opposed to angiography, which displays a projection. Compared with angiography, ultrasound may offer the additional advantage that clinicians are not rushed through procedures by a desire to keep exposure times to a minimum.
Conventional two dimensional (2D) ultrasound is routinely used to guide liver biopsies, with the needle held in a “guide” attached to a transducer. The guide keeps the biopsy needle in the plane of the image while the tip of the needle is directed to targets within that same plane. This system typically requires a clinician to look away from his hands at a video monitor, resulting in a loss of direct hand-eye coordination. Although the clinician can learn this less direct form of coordination, the natural instinct and experience of seeing one's hands before one's eyes is preferred.
As a further disadvantage, the needle-guide system constrains the biopsy needle to lie in the image plane, whereas the clinician may prefer the needle to intersect the image plane during some invasive procedures. For example, when inserting an intravenous (IV) catheter into an artery, the optimal configuration may be to use the ultrasound image to visualize the artery in cross-section while inserting the needle roughly perpendicular to the image into the lumen of the artery. The prior art system just described may not be capable of accomplishing this task.
A related visualization technology has been developed where three dimensional (3D) graphical renderings of previously obtained CT data are merged with an observer's view of the patient using a partial or semi-transparent mirror, also known as a “half-silvered” mirror. A partial mirror is characterized by a surface that is capable of both reflecting some incident light as well as allowing some light to pass through the mirror. Through the use of a partial mirror (or other partially reflective surface) a viewer may see an object behind the partial mirror at the same time that the viewer sees the image of a second object reflected on the surface of the mirror. The partial mirror-based CT “Image Overlay” system requires independent determination of location for both patient and observer using external 6-degree-of-freedom tracking devices, so as to allow appropriate images to be rendered from pre-acquired CT data.
Another recently developed imaging technology merges ultrasound images and human vision by means of a Head-Mounted Display (HMD) worn by the human operator. The location and orientation of the HMD is continuously determined relative to an ultrasound transducer, using 6-degree-of-freedom tracking devices, and appropriate perspectives of the ultrasound images generated for the HMD using a graphics computer.
These prior art systems may not be appropriate for use with a practical real-time imaging device. Controlling the multiple degrees of freedom can be difficult, and the systems may have too many complex parts to be useful. As such, there is recognized a need in the art to provide a device capable of merging a human's normal vision of an object with an “internal” image of the object that emphasizes freedom of operator movement and/or simplicity of design.